The present invention relates to mobile radio telecommunication systems. More particularly, and not by way of limitation, the present invention is directed to a method of adjusting uplink transmit timing at each mobile station that enables synchronous reception at a base station.
In mobile radio telecommunication systems, propagation delays for transmissions from mobile stations vary due to differences in the distance of each mobile station (MS) from the receiving base station (BS). In many systems, such as the Global System for Mobile Communications (GSM), signals received at the BS are synchronized by providing periodic adjustments in transmit timing, known as timing advance (TA) values, to each MS. In other systems, especially Frequency Division Duplex (FDD) Code Division Multiple Access (CDMA) systems such as Wideband CDMA (WCDMA), no effort is made to synchronize uplink transmissions at the chip level, although coarse synchronization at the frame level is maintained. However, in some CDMA systems such as the Time Division Duplex Synchronous CDMA (TD-SCDMA) system being developed in China, an effort is made to synchronize transmissions on the uplink through the use of periodic transmit time adjustments for each mobile station. The TD-SCDMA system uses special codes designed to preserve orthogonality against multipath echoes within a certain time window around the time of reception of the signal at the BS, and hence can benefit from uplink synchronization. Uplink synchronization is a requirement, in general, for any TDD system regardless of its multiple access method.
As described above, prior systems have attempted to solve the problem of synchronizing received signals at the BS by periodically transmitting adjustments to each MS to adjust its transmit timing. For the first uplink transmission in a session, transmit timing is determined based on receive timing on the downlink. In some systems, such as GSM, a pre-defined offset from the time of reception of a slot on the downlink is used. In other systems, such as the TD-SCDMA system, the received power on the downlink slot is used to determined the offset between the transmit time and the receive time. The timing control for the first uplink transmission may be referred to as open-loop timing control. Once the MS starts transmissions, the BS monitors the time of reception of the signal transmitted by each MS. If a significant deviation from the required time of reception occurs for a given MS, a command to adjust transmit time is sent to that MS. This timing control may be referred to as closed-loop timing control.
There are several disadvantages of the existing solutions. A first disadvantage is that they require periodic control commands to the MSs to ensure uplink synchronization. In mobile radio telecommunication systems with larger cells and/or a large number of highly mobile MSs, this can result in a significant increase in signaling load. It would be advantageous to have a method of adjusting transmit timing at each MS, while minimizing the necessity for transmit time updates from the BS. A second disadvantage is that the initial estimate of transmit timing made during the open-loop timing control phase is not as accurate as desired. It would be advantageous to have a method of adjusting transmit timing at each MS that improves the initial estimate of transmit time during the open-loop timing control phase. The present invention provides such a method.